Many space-based telescopes and spectrometers require ultralow read noise in order to observe a large number of astrophysical phenomena associated with galactic and stellar evolution, high red-shift objects, etc. Detection of ultralow photon flux levels are also required in a large number of environments involving tactical and strategic military applications, such as night vision. Detection of faint objects require either extremely long integration times to build enough signal to be above the system noise floor, or image intensification using photo-multiplier tubes or micro-channel plates (MCP). Both the photomultiplier approach and the MCP approach suffer from the ungainly requirements of large mass, high voltage, high power, high dead-times, small dynamic range and in the case of MCP, "scrubbing" for stability.
Flicker noise often limits the exposure time in conventional IR imaging systems, thus limiting detectability of ultralow level IR images. In a typical IR imaging system, the analog nature of the image signal integration and readout processes makes it susceptible to noise pick-up along the entire path of the image signal processing. The multiplexer noise, consisting of white noise in MOS transistors and unwanted clock pick-up, is typically around 10-20 electrons in low-noise systems. Multiplexers with subten-electron read noise are far and few between, and tend to suffer from large response nonuniformity and nonlinearity.
Detection of faint objects will be greatly enhanced by providing for readouts of signals from pixel cells with subelectron read noise. An object of this invention is to provide ultralow noise sensors in an array of pixel unit cells at a focal plane in which limitations due to read noise can be overcome by counting photons received within each pixel unit cell, and generating a one-bit digital signal from photons received per clock-pulse for integration by counting and making the readout system virtually noise free.
The array of unit-cell sensors may have a hybrid structure, similar in that respect to conventional IR sensors, with an important difference, besides the unit-cell realization itself, that the readout system comprises a novel multiplexer that is sensitive to single photoelectron signals. Such a hybrid solid-state sensor structure enables on-chip photon counting to take place directly for integration, thus greatly enhancing the capability of ultralow light level image detection.
On-chip digitization has been previously demonstrated [B. Fowler, A. El-Garnal, and D. Yang, "A CMOS area imager sensor with pixel level A/D conversion," Digest of Technical Papers, 1944 IEEE International Solid-State Circuits Conference, Vol. 37, pp. 226-227 and U.S. Pat. No. 5,461,425] but as realized was limited to detection of large signal fluxes, and was not amenable for solid-state photon-counting. Instead the image sensor was realized with phototransistors whose high conductivities are related to a high level of light. The analog signals thus generated at the phototransistors were converted to a serial bit stream by an on-chip A/D converter. That on-chip A/D approach is thus clearly useful only for normally high photon flux levels.
Another on-chip digitization System had been suggested earlier [U.S. Pat. No. 4,710,817] for a solid-state image sensor in which the photon flux at each pixel could be integrated digitally by an electronic digital counter. The readout process then would involve transfer of digital signals that are not as susceptible to noise and radiation interference as analog signals. Furthermore, a conventional analog-to-digital converter would not be required, which is another source of signal degradation. However, the photodetectors there were selected to be avalanche photodiodes or microchannel plates (MCP) which suffer from the ungainly requirements noted above in order to provide photoelectric signals of sufficient amplitude to drive the digital counters through conventional buffer amplifiers. That system was evidently not intended for use in ultralow flux level conditions where photons are generally received individually with an incidence of two or more arriving at about the same time being statistically so small during an integration period as to not distort the image over the short period of time they are individually counted. Instead, the flux level contemplated was so high that such incidence would be statistically high. In an attempt to improve contrast, the system was so designed that each photoelectric signal pulse created by a group of photons be converted to a number of distinct pulses in proportion to the pulse peak value and time of duration using a signal level detector and pulse generator circuit to inject into the integrating counter a proportional train of distinct pulses. That form of analog to digital conversion is at best an approximation of the true image photon flux and is clearly intended for use in relatively high photon flux conditions.
More discriminating PIN photon-flux detector arrays are commercially available for UV/visible photon-counting imaging devices from Hughes Technology Center (HTC), but they use conventional CCD architecture based on analog charge integration and analog pixel charge multiplexing for readout before analog-to-digital conversion. Thus, the HTC imaging device has much higher noise (&gt;50 electron rms) than can be tolerated for many applications that require imaging under low level photon flux conditions. Consequently, such PIN photon flux detector arrays are not useful for many scientific and commercial applications that require accurate ultralow photon flux level detection.
Several astrophysics-missions with space-telescopes and spectrometers for the IR band have been planned by the National Aeronautics and Space Administration or are already in use. For example, in astronomy and astrophysics, infrared images of objects have led to discovery of several features that are hidden in other spectral bands using conventional CCD detectors. Satellites with IR detector arrays are being planned to explore temperatures in the upper atmosphere, conduct surveys of terrestrial minerals, water and agriculture, and record weather patterns. What is now needed is an array of digital-counting pixels for imaging in ultralow photon flux level conditions that prevail in some situations.
Medical researchers also use IR detector arrays as tools to evaluate skin diseases, circulatory and neurological disorders, breast cancer and neo-natal birth. IR detector arrays also have potential applications in industrial robotics, and are being used for industrial thermography (mechanical and electrical fault detection), high temperature and chemical process monitoring, spectroscopy, night vision and materials research. Some of these uses also require imaging under ultralow photon flux level conditions.
Because of these situations requiring ultralow photon flux level detection, reference will sometimes be made to an infrared focal-plane array (IRFPA) in the description of preferred embodiments of the present invention. However, other applications will require UV/visible detector arrays also sensitive to low photon flux level detection. Consequently, it is not intended that the concept of the invention be limited to infrared radiation. Thus, for "IR" in IRFPA, "UV/visible" FPA is to be appropriately assumed equivalent in the context that it is used. Similarly, wherever FPA is used, the reference to FPA is to be understood to be generic to IR and UV/visible focal-plane arrays since the present invention is suitable for use in other wavelength bands requiring ultralow photon flux level conditions that would require only the proper selection of the photoelectric conversion material to be used in the pixel array at the focal plane of an optical lens for the wavelength band of interest.
Like focal-plane arrays operating in UV/visible spectral bands, large IRFPAs are also required to operate with severe power dissipation, real estate and throughput constraints. Typical dimensions of an IRFPA readout unit-cell are 50 .mu.m.times.50 .mu.m in area, and typical maximum power dissipation is 100 .mu.W/pixel. The low power dissipation requirement also imposes a constraint on the kind of detector that can be used. State-of-the-art IR detectors are photoconductive or photovoltaic detectors. Photo-conductive detectors require a quiescent current for operation, thus increasing focal plane power dissipation. On the other hand, a photovoltaic detector is essentially a reverse-biased diode requiring very low quiescent current for operation. Consequently, for low power, staring focal-plane arrays, a photovoltaic detector diode is preferred.
There are two major differences between UV/visible and infrared imaging focal-plane arrays. First, silicon, the most familiar and best understood photoelectron conversion material, cannot be easily used for detection of infrared radiation. This is because silicon has a bandgap energy of 1.12 eV. Therefore, a photon whose energy is less than 1.12 eV will not generate an electron-hole pair in a silicon photovoltaic detector, thereby preventing its use for detection of IR radiation. Thus, it can be seen that IRFPAs operating at 3-5 .mu.m and 8-12 .mu.m bands, require detector materials having band gap energy of 0.25 eV and 0.1 eV, respectively. In the absence of silicon photovoltaic detectors at these wavelengths, photovoltaic detectors are built on narrow band gap materials such as IV-VI compounds (lead salts), II-VI semiconductors (mercury salts), III-V semiconductors (indium and gallium salts). IR detectors are built on indium antimonide (InSb), a III-V compound, and mercury cadmium telluride (HgCdTe), a II-VI compound. IR detection in silicon is carried out at 3-5 .mu.m bands by using a platinum silicide (PtSi) Schottky barrier diode (SBD) fabricated in an a-Si layer.
In SBDs, internal photoemission is responsible for exciting a photocurrent across the relatively small Schottky barriers. The quantum efficiency of PtSi in a SBD is extremely low, being in the range of 1-2%. The quantum efficiency can be increased somewhat by changing the thickness of PtSi or by using an alternate metal film such as Pd.sub.2 Si. However, the dark current is also increased as a result, often resulting in degradation of performance rather than an improvement. Other novel IR detector technologies exist, but all are constrained by the absence of efficient low-noise readout and multiplexer structures.